Modernism, in its broadest definition, is modern thought, character, or practice. More specifically, the term describes both a set of cultural tendencies and an array of associated cultural movements, originally arising from wide-scale and far-reaching changes to Western society in the late nineteenth and early twentieth centuries. The term encompasses the activities and output of those who felt the "traditional" forms of art, architecture, literature, religious faith, social organization and daily life were becoming outdated in the new economic, social and political conditions of an emerging fully industrialized world.

Modernism rejected the lingering certainty of Enlightenment thinking, and also that of the existence of a compassionate, all-powerful Creator. This is not to say that all modernists or modernist movements rejected either religion or all aspects of Enlightenment thought, rather that modernism can be viewed as a questioning of the axioms of the previous age.

A salient characteristic of modernism is self-consciousness. This often led to experiments with form, and work that draws attention to the processes and materials used (and to the further tendency of abstraction). The poet Ezra Pound's paradigmatic injunction was to "Make it new!" Whether or not the "making new" of the modernists constituted a new historical epoch is up for debate.

Current interpretations of modernism vary. Some divide 20th century reaction into modernism and postmodernism, whereas others see them as two aspects of the same movement.

The first half of the nineteenth century for Europe was marked by a number of wars and revolutions, which contributed to an aesthetic "turning away" from the realities of political and social fragmentation, and so facilitated a trend towards Romanticism: emphasis on individual subjective experience, the sublime, the supremacy of "Nature" as a subject for art, revolutionary or radical extensions of expression, and individual liberty. By mid-century, however, a synthesis of these ideas with stable governing forms had emerged, partly in reaction to the failed Romantic and democratic Revolutions of 1848. It was exemplified by Otto von Bismarck's Realpolitik and by "practical" philosophical ideas such as positivism. Called by various names-in Great Britain it is designated the "Victorian era"-this stabilizing synthesis was rooted in the idea that reality dominates over subjective impressions.

Central to this synthesis were common assumptions and institutional frames of reference, including the religious norms found in Christianity, scientific norms found in classical physics and doctrines that asserted that the depiction of external reality from an objective standpoint was not only possible but desirable. Cultural critics and historians label this set of doctrines realism, though this term is not universal. In philosophy, the rationalist, materialist and positivist movements established a primacy of reason and system.

Against the current ran a series of ideas, some of them direct continuations of Romanticism schools of thought. Notable were the agrarian and revivalist movements in plastic arts and poetry (e.g. the Pre-Raphaelite Brotherhood and the philosopher John Ruskin). Rationalism also drew responses from the anti-rationalists in philosophy. In particular, Hegel's dialectic view of civilization and history drew responses from Friedrich Nietzsche and Soren Kierkegaard, who were major influences on existentialism. All of these separate reactions together began to be seen as offering a challenge to any comfortable ideas of certainty derived by civilization, history, or pure reason.

From the 1870s onward, the ideas that history and civilization were inherently progressive and that progress was always good came under increasing attack. Writers Wagner and Ibsen had been reviled for their own critiques of contemporary civilization and for their warnings that accelerating "progress" would lead to the creation of individuals detached from social values and isolated from their fellow men. Arguments arose that the values of the artist and those of society were not merely different, but that Society was antithetical to Progress, and could not move forward in its present form. Philosophers called into question the previous optimism. The work of Schopenhauer was labelled "pessimistic" for its idea of the "negation of the will", an idea that would be both rejected and incorporated by later thinkers such as Friedrich Nietzsche.

Two of the most significant thinkers of the period were, in biology, Charles Darwin, and in political science, Karl Marx. Darwin's theory of evolution by natural selection undermined the religious certainty of the general public, and the sense of human uniqueness of the intelligentsia. The notion that human beings were driven by the same impulses as "lower animals" proved to be difficult to reconcile with the idea of an ennobling spirituality. Karl Marx argued there were fundamental contradictions within the capitalist system-and that the workers were anything but free. Both thinkers would spawn defenders and schools of thought that would become decisive in establishing modernism.

Historians have suggested various dates as starting points for modernism. William Everdell has argued that modernism began with Richard Dedekind's division of the real number line in 1872 and Boltzmann's statistical thermodynamics in 1874. Clement Greenberg called Immanuel Kant "the first real Modernist", but also wrote, "What can be safely called Modernism emerged in the middle of the last century-and rather locally, in France, with Baudelaire in literature and Manet in painting, and perhaps with Flaubert, too, in prose fiction. (It was a while later, and not so locally, that Modernism appeared in music and architecture)." At first, modernism was called the "avant-garde", and the term remained to describe movements which identify themselves as attempting to overthrow some aspect of tradition or the status quo.

Separately, in the arts and letters, two ideas originating in France would have particular impact. The first was impressionism, a school of painting that initially focused on work done, not in studios, but outdoors (en plein air). impressionism paintings demonstrated that human beings do not see objects, but instead see light itself. The school gathered adherents despite internal divisions among its leading practitioners, and became increasingly influential. Initially rejected from the most important commercial show of the time, the government-sponsored Paris Salon, the Impressionists organized yearly group exhibitions in commercial venues during the 1870s and 1880s, timing them to coincide with the official Salon. A significant event of 1863 was the Salon des Refuses, created by Emperor Napoleon III to display all of the paintings rejected by the Paris Salon. While most were in standard styles, but by inferior artists, the work of Manet attracted tremendous attention, and opened commercial doors to the movement.

The second school was symbolism, marked by a belief that language is expressly symbolic in its nature and a portrayal of patriotism, and that poetry and writing should follow connections that the sheer sound and texture of the words create. The poet Stephane Mallarme would be of particular importance to what would occur afterwards. At the same time social, political, and economic forces were at work that would become the basis to argue for a radically different kind of art and thinking. Chief among these was steam-powered industrialization, which produced buildings that combined art and engineering in new industrial materials such as cast iron to produce railroad bridges and glass-and-iron train sheds-or The Eiffel Tower, which broke all previous limitations on how tall man-made objects could be-and at the same time offered a radically different environment in urban life.

The miseries of industrial urbanism and the possibilities created by scientific examination of subjects brought changes that would shake a European civilization which had, until then, regarded itself as having a continuous and progressive line of development from the Renaissance. With the telegraph's harnessing of a new power, offering instant communication at a distance, the experience of time itself was altered.

Many modern disciplines (for example, physics, economics, and arts such as ballet and architecture) denote their pre-twentieth century forms as "classical." This distinction indicates the scope of the changes that occurred across a wide range of scientific and cultural pursuits during the period.